The invention relates to an FM-receiver comprising an FM-channel coupled to an aerial input, the FM-channel comprising in succession an RF-input stage, a mixer stage, an IF-circuit and a ratio detector, the ratio detector incorporating silicon diodes for detection of an FM-signal in said FM-channel as well as a smoothing capacitor which is conductively connected to said diodes for a dynamic amplitude limitation of the ratio detector output signal in dependence on the amplitude of the ratio detector input signal.
Such an FM-receiver is employed in the Philips television receiver type R 17 B 720 and is published in printed form in the manual thereof.
In practice, an unwanted amplitude modulation of the FM-signal to be processed in an FM-receiver may be produced due to inter alia transmission noise, multipath reception, a disadvantageous transmitter location, frequencies of unwanted transmitters close to the carrier frequency of the desired signal then occurring, edge detection on the side edges of the IF-bandpass response, etc. In the known FM-receiver this unwanted amplitude modulation is predominantly suppressed at large input signals by amplitude limitation of the output signal of the FM-circuit and at small input signals by integration via the smoothing capacitor of the ratio detector.
The amplitude of the input signal at which the limiting action in the amplifier/limiter stages of the IF-circuit starts determines the input sensitivity of the FM-receiver. If this amplitude is small then the input sensitivity is high and vice versa. The ratio detector of the prior art FM-receiver provides a low input sensitivity while maintaining a suppression of amplitude variations at also those input signals at which the amplifier/limiter stages of the IF-circuit are not limiting. Consequently, the prior art FM-receiver is rather insensitive to interferences owing to, for example, noise and/or rapid field strength fluctuations, also when small signals are received, which is of particular importance in mobile FM-reception. It also shows a non-agressive tuning behaviour without the need for additional circuits, such as, for example, a muting circuit.
In the present state of the art it is, however, costly and complicated to realize adequately small signal processing in a ratio detector. In the ratio detector of the known FM-receiver use is made of silicon diodes as detection diodes. As will be known, silicon diodes have a threshold voltage which, depending on inter alia the configuration and the diode surface area, may be between approximately 450 and 700 mV. In order to obtain a correct detection of FM-signals having an amplitude smaller than this threshold voltage the silicon diodes of the ratio detector must remain in the conductive state also at such small signal amplitudes. In the known FM-receiver this is realized by means of a bias circuit which applies a diode biasing current to said silicon diodes. Such a biasing circuit is, however, costly and complicated. In addition, the diode bias currents introduce in the tuned state an unwanted d.c. voltage component in the output voltage of the ratio detector, as a result of which this output voltage is not suitable for AFC.
It is also known to use germanium diodes as detection diodes. Since germanium diodes exhibit a much lower threshold voltage than silicon diodes the use of a biasing circuit is not necessary. However, germanium diodes are much more expensive than silicon diodes. In addition, in contrast with silicon diodes, they are not integrateable.
It is an object of the invention to provide an FM-receiver which compared with the prior art FM-receiver can be realized in a simpler and cheaper manner and has at least the same advantageous properties.